The term ‘MEMS’ (Micro-electromechanical system or structure or switch) can encompass various devices. A common arrangement of a MEM device comprises a freestanding beam with a first electrode located opposite a second electrode. The first and the second electrode are mutually separated by an air gap. The first electrode can be moved towards or away from the second electrode by application of an actuation voltage to provide an electrostatic force (in principle other forces could be used such as an inductive force).
Some common applications are:
use as a microphone or a loudspeaker; (using a specific type of membrane)
use as a sensor, particularly an air pressure sensor
use as a resonator
use as pixel switches in a display, or driving a mirror for an optical switch,
use in RF applications, particularly as a switch or as a variable capacitance.
One of the commercially important applications is the use for variable impedance matching with integrated band switching in the front end of a mobile wireless device such as a phone or computer.
Two common constructions are as follows:
1. A MEMS structure in a substrate of silicon. In this case the electrodes are oriented perpendicular to the substrate surface. This construction is used for the sensor application and for the resonator application (other applications are not excluded).
2. A MEMS structure as a thin-film element. The beam is here oriented substantially parallel to the substrate. This type of MEMS structure is used for RF MEMS. There are at least two constructions for the beam:
a double clamped beam (a beam that is connected to the substrate surface at two or more sides, so the deflection to the substrate occurs in the centre of the beam). This type of beam is known as a beam with both ends built in and is statically indeterminate.
a single clamped beam (in which case the deflection to the substrate occurs at the end of the beam). This type of beam is called a cantilever beam and is statically determinate.
The beam is generally provided with holes, provided for the etching of the sacrificial layer between the beam and the substrate to create the air gap. These holes also help to reduce air damping by allowing air to flow in and out of the cavity between beam and substrate, while opening or closing the beam. However there are manufacturing techniques in which the beam is assembled to the substrate, so no holes are needed for the etching as described in GB-A-2,353,410. It is also possible to use a beam that is an intermediate layer between a top electrode and a bottom electrode.
It is known from U.S. Pat. No. 5,955,659 to provide holes in a movable diaphragm element of a MEM pressure sensor device, to vent fluid from a cavity. It is known from US patent application 2003/0148550 to provide a MEM device with holes in the movable element and having a copper electrode recessed into the substrate to reduce stiction. Another method of addressing stiction is shown in WO 01/59504, using a flap valve to force fluid such as a gas through holes in the substrate, into the contact area. It is also known to provide roughness or dimples on the contact surfaces to reduce stiction.